November 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY USES
Since silicoiie finishes are so new and facilities for producing them are limited, they have not yet been extensively used. Consequently any discussion of their applications must be in the nature of prediction. Until their cost is reduced by volume production, they will be confined t o special uses where heat, chemical, and, possibly, weather resistance are essential. Finishes for kitchcn ranges, stoves, furnaces, boilers, motors, hot exhaust
1379
stacks, and chemical plant equipment are examples. Khen cost# becoinc competitive through increased production or by modification with alkyd resins, there is a long list of possible uses, such as for finishing automobiles, refrigerators and other home appliances, hospital equipment, signs, metal furniture, g a d i n c pumps. farm niachinery, and many other products. R ~ ~ ~ i T: , r ~i l 5\, 19-17
POLYSILICIC ACID ESTERS..
..
Preparation from Sodium Silicate R. E. I . D U P O N T
DE
K. ILER
N E M O U R S & C O M P A N Y . INC.. C L E V E L A N D . OHIO
P.S . P I N K N E Y E. I . D U P O N T D E N E M O U R S 8c C O M P A N Y . INC.. W I L M I N G T O N . D E L .
I31 TR4XSFERRIXG low-molecular-weight silicic acid, prepared by acidification of sodium silicate, from aqueous solution to n-butyl alcohol, it has been found that partial esterification can be effected by dehldration through azeotropic distillation. The partially esterified poly silicic acid, containing from 0.5 to 0.6 butoxy group per silicon atom, is a resinous solid, soluble in organic solFents and stable o\er long periods of time in butyl alcohol solution a t a concentration of ZOY' SiOo. Compatibility with hydrocarbon soltents and stability toward gelling increase with the degree of esterification. A preliminary biscosity stud, indir-ates that the partially esterified poly silicic acid molerules ma? he spherical in shape.
silicate until t h i s esterification is finally effected. Fur exnniple the acidification of the solution of sodium silicate must be carried out under conditions 15-hichgive silicic acid of relatively loll- molecular weight-that is, at a low temperature with rapid mixing tcv obtain a relatively dilute solution having a final p H of 1.7 Transfer of silicic acid to the alcohol-for example, tert-butyl alcohol-must then be effected before polymerization has reachen thc gel stage. So long as the alcohol solution contains a n appreciable amouni of water, esterification does not, proceed to an extent mfficicnt provide a product nhich is stable on long st,orage. In order t r obtain a product of minimum molecular weight, watrr is promptlj removed from the alcohol solution as rapidly and a t as low B temperature as possible by azeotropic distillation under reduced pressure. .is esterification proceeds and the silicic acid becomeHI.; possibility of preparing a highlv rvactivc. I o K - I I ~ ~ ~ I c ~ L I more I ~ ~ stable toward polymerization, tho distillation temperature can be a l l o w d t o rise through ( a ) a n increase in pressure, (bj n-eight silicic acid in aqucous solution by thc acidification of substitution of a second higher boiling alcohol for that used in the sodium silicate has been known for n k i y years. 3lyliua and extraction, or (c) both. I n the last, stages of the estcrification Groschuff ( 8 ) found that, by carefully neutralizing a solution of the distillation is carried out under atmospheric pressure to sodium silicate in the cold with hydrochloric acid, a solution of effcct removal of ivater continuously as i t is formed. During ali qilicic acid is obtained xvhicli passe.? freely through a dialyzing hut the last stage of the esterification process the concentration ot membrane. Tourky (IO) visualized that freshly formed silicic silicic acid, expressed in terms of Si02, is kept below 10% by ocacid in aqueous solutions is in an actives condition capable casional addition of alcohol, cithcr that used for the extraction or of linking with othcr active molecules. Killstiitter ( 1 1 ) has a higher boiling alcohol of nhich the ester is desired. The condhon-n that silicic acid can exist in aqueous solution in the monocentration of the resulting substantially anhydrous solution of meric form for a short time, provided a small amount of hydropolysilicic acid estcr is adjusted to about 20% SiO,. I n this form chloric acid is present as a stabilizing agent. the ester, xhich contains up to about 0.6 alkoxy groups per silicon Holvcver, such silicic acid of Ion- molecular weight is su UTIatom, is stable tonard gelling a t ordinary temperatures over B stable in aqueous solutions that any attempt to isolate it by period of several years. evaporation of water, even at ordinary temperature, results in For practical purposes tert-butyl alcohol is the most satisfactor) rapid polymerization to a gel. The direct csterification of silicic alcohol for usc as the silicic acid crtractant. During the esteriacid has t,herefore remained impractical until the recent discovery of a method for transferring silicic acid of low molecular neight fication it is generally replaced by the higher boiling n-butyl alcohol, nhich makes possible more rapid removal of m t c r from from aqueous solution t o solution in a n alcohol. This transfer is the systcni. Solvent-free butyl polysilicate containing 0.5-0.6 accomplished by extraction of the acid with a suitable alcohol (6). Esterification is then effected by azeotropic distillation of water butoxy group per silicon atom is a somewhat tacky, resinous from the alcohol solution (4). material which is readily soluble in many organic solvents, including choroform, acctonc, and benzene. However, the esteriThe success of this technique depends upon the careful choice of fication of the silicic acid is apparently incomplete, since the solconditions t o keep the degree of polymerization within suitable vent-free product slowly becomes insoluble if permit,ted to stand limits from the moment the silicic acid is generated from sodium
T
t(1
1380
'
INDUSTRIAL AND ENGINEERING CHEMISTRY
ior several days at ordinary teniperatui'es. T h e prewrice of unesterificd acid groups is also indicated by the clieniicnl :inalysix of the product after it has been dried in a thin film under rcduced pressure. Less completely esterified intermediate products arc soluble i n alcohols, ac(tioiie, or other polar solvents, and are precipitated hy the addition of benzene and other hydrocarbons. Cpon evaporation of the solvent these products deposit a hard, brittle film which cannot be redissolved. Polysilicic acid esters of higher boiling alcohols, such as ortyl, dodecl-1, cyclohesyl, stearyl, and methallyl alcohols, can he prc.pared conveniently from butyl polysilicate by a n ('Stcar interrhange reaction. T h e preparation of organic solverit-soluble, partially csterified polysilicic acid from sodium silicate not only sugg riicthod of approach for obtaining a better undcxistan chemistry of silicic acid but also opens u p many practical posribilities involving the incorporation of silica, derived from >odium silicate, into organic product?. For example, thcse produrts can be incorporated into alkyd resin finishes t o improve -cd with other typei: of liardiltlss and rate of drying ( 3 ) or i~iyinsto yield clc:ir, hard, abrasion-rt t a n t f i l l l l ~uscful as pl,i1tc.(.tiv,,surf:ici, roatingi: on organic 1)lantics ( I ) , LOX'-.\lOLECULAR-R EIGHT SILICIC ACID
-4 study oi tlic r,atrs of polymtrinstio~iof silicic acid in aqueous solution has shown that this material is least stablc in aqueous mlution in the pH range 5 t o 7 , and most stable at, a p H of about, 1.5 to 2.0. Therefore, in order to liberate silicic acid of loncst possible molecular weight, it is cssential to carry out tlie neutralization in such a manner that tlie solution is conwrted praet ically instantaneously froni t h t s alkaline condition t o a pH uouiid 1 to 2.5. This is most h i p l ? - effcctcd by passing a dilute ,elution of sodium silicate in a thin stream into a violently agitated solution of dilute acid, the tcniprlrature preferably heiiig iiiaintained helo\r 30" C., using surh an e s w g s of acid that, the final p H does not rise above 2.0.
Vol. 39, No. 11
polar groups is prcseni, saturation of the aqucxous phase witti a salt such as sodium chloride brings thc silicic acid into a Ino.;~~ association with the organic solvent, so t,hat a separatc liquid phase is formcd ll-hich can be mechanically separated from tho Ixinc layer. A possible explanation is that a hydrogen bond is formed betn-ecn the hydrogen of the silanol group (-SOH) arid the electron donor atoms, such as gen or nitrogen, iii t h o polar organic solvent. Solvents of EL- hydrogen-bonding agents. Eqters, amides, ketones, and alcohols can all function as extravtion solvents in this n.ay (,?, 6 ) . However, where the silicic acid is to be esterified, it is preferable to use a n extracting solvent which has a relatively lo^ boifiiig point and which can therefore be readily rc.cover.ed from the final esterificd product. For this purpohii tp,.l-hutyl alrohol appcws t o be one of th? most satisf:tcto1,y.
Immediately after the preparation of ttic 3-120 w, of j'licic acid solution already described, 1357 cr. (1070 grams) of tertbutyl alcohol are added. The mixture is tirred for 15 minut,es and then permitted to stand for 16-18 hours a t room temperature (below 30" C.) in order to ensure niasinium yield. U p to this point the solution is coniplete1)- homogeneous. Hot\ adding 1017 grams of sodium chlorid: and stirring for an alcohol-rich liquid phase appears containing silicic mixture is permitted t o stand for 20 minutes in a separatory funnel: the loner, aqueous ssline layer is dran-ii off: and th(1 upper, alcohol layer is sc't asid(. for furthcr proctssiiig. As t,he separation of the two layers is sometimes inconiplete because of emulsification, it is advisahle t o add 24 cc. of a 2 5 gelatin solution prior to the addition of salt. The gelatin apparently brings about coagulation of a small amount of unidentified material rrhich is responsible for the formation of a I n a typical esperiment perfornicd in this manner alcohol layer (1540 cc.) contained 1 4 . 0 7 silica, 0 chloride, and 10-ljcc n.at,er, as determined by titration with the Fischer reagent, (9). The yield of silica is about'80-85% of that originally present in the sodium silicate. IIEL4TION BETWEEY MOLECULAR W E I G H T A N D YIELD
I t i h t x stsparation of thcs alcohol phasi, is coniplt)te, thc yield in pt,iniarily dependent upon the molecular weight of thv silicic dilute solutions of sodium silicate and sulfuric acid are prepared iwid i t 1 thc aqueous solution at the time of extraction. -1lthougti as folloys: Sodiuni silicate, Grasselli S o . 20 \ I T grade (:I cornt h c ~ r c i.~ litrle or no change in the appearance of the aqucious mercial n a t e r glass containing 28.10C; SiOI and having a m i g h t silicic acid solution, thew is :L ratio of SiO? S a ? O = 3.25) is diluted in the proportion of 777 grams progresiivc, increase in thv inolccof sodium silicate (liquid) t o 1138 ular rreiglit of the silicic acid as tlic A. GELATIN ONLY, N O S ' L I C I C A C I D grams ot 11-ater, a total of 1913 a , VERY F X S Y , DILUTE SOL solution is agcd, the ratc iicxitig s grams or 1710 cc. Separately 1710 C DE. SOLS O F INTERMEDIATE AGE F: 'SOL EECOMIUG SLIGHTLY CLOLDY function of the concentratioti of cc. of 7 . 3 5 5 culfuric acid are preG. SOL AT VERY SOFT G E L STAGE pared and cooled t o 20" C. Tlie silicic acid, the pH, arid the ti'nidiluted sodium silicate solution is pcritt UI'C. then run into the equal volume of -4n empirical titration rnc~thotl dilute acid, IYith violent agitation. has been dcw:lopcd t o f o l l o ~this The addition is made in a stream not over 0.1 inch in diameter added polymerization ( 2 ) . This me1 hod directly to the r o r t e s created by depends upoti the discovery th:it the stirrer, and is complete in about the precipitntion of soluhlt: silicic 5 minute?. The pH of the resulting acid by gelatin is inhibited by vxter3120 cc. of si icic acid solution is 1.7 * 0.05, T h i i solution is then ready miscible organic solvents coiitainfur extraction of the polysilicic ing electron-donor groups-for esacid by an alcohol. ample, the diethyl ether of diethylene glycol (diethyl Carbitol). E X T R A C T I OFSR O N A ~ r ~ o r - i However, the concentration of thtx SOLUTIOSTITH POLARORG-LSIC latter organic solvent required t o SOLVENTS.J. S. Kirk ( 5 ) appears inhibit precipitation depends upon to be the first t o have discovered the concentration of electrolyte in that, b y a combined salting-out the mixture and upon the molecular and extraction process, soluble silicic w+it of silicic acid. The precipiacid can be transferred from a n tation point is relatively independaqueous solution to organic solvents. ent of the concentrations of geIatin Silicic acid alone cannot be salted and of silicic acid. The test is out of aqueous solution; if, hov,-Figure 1. Titration Curves of Silicic Acid carried out as follows: Solutions of Different RIolecular Weights cver, a n organic solvent containing
PRE~~AR.ITION. The follon-ing example is illustrative. Separate
November 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
Ten cubic centimeters of a solution of silicic acid, free from organic solvents and containing from 4 4 % SiO? at, a p H of 1.5 t o 2.5, are pipetted into a 1 X 8 inch test tube. T o this are added 2 or 3 cc. of a n aqueous solution of diethvl ether of diethylene glycol (diethyl Carbitol) containing 500 grams per liter, the pH heing adjusted t o 2.5 x i t h hydrochloric acid. T o this are added 5 cc. of a 2% solution of high grade edible gelatin (Iinox Super XXX). Sufficient diethyl Carbitol solution is used t o prevent t,he formation of a precipitate when the gelatin solution is added. This mixture is then titrated x i t h a standard solution of sodiuni chloride containing 300 grams per liter (previously adjust,ed t o pH 2.5 n-ith h)-drochloric acid) until the solution becomes turbid. This is observed by holding the tube against an aperture ', inch in diameter illuminated from behind by a 60-watt lamp. T h e end point is arbitrarily taken when the clear outline of the aperture can no longer be discerned. This relatively crude method is satisfactory, since the end point is quite sharp; the mixture usually changes from perfect transparency t o extreme turbidity within 0.2 t o 0.3 cc. The mixture is shaken vigorously for about, 10 seconds after each addition of salt solution before the turbidity ved. T h e turbid solution is then cleared up by adding another 1 or 2 cc. of the diethyl Carbitol solution and again titrated t y the end point with sodium chloridcx solution. At each end point the total concentrations, in terms of grams per 100 cc., of sodium chloride and of diethyl Carbitol are calculated with the total voiunie a t each end point taken into account. These values are plotted on rectangular graph paper with the sotliuni chloride concentration as ordinate and the concentrationof diethyl Carbitol as abscisa. The end points will be found to lie on a straight line, which can be extrapolated to the diethyl Carhitol asis. Such plots are shown in Figure 1. If distilled water, adjusted t o p H 2.5 with hydrochloric acid, is used instead of the silicic acid solution. a similar precipitation of gelatin alone is observed. Thi.3 precipitate is solubilized by diethyl Carbitol, and the cnd points obtained on titration with 5alt solution similarly fall on a straight line. As shown in Figure 1, the position of the lines obtained n.ith sols of increasing niolecular weight move progressively t o the right-that is, higher concentrations of diethyl Cartiitol are required to prevent precipitation. The intercept on the diethyl Carbitol axis therefore wrvcs to indicate the relative molecular weight of the silicic acid. I n order to have an arbitrary scale with positive values, a function S n-as adopted such that
1381
the addition of alcohol and ialt, the following results were obtained : tert-BuOR Used/I,. Silicic Acid Soln.
s
200
eilicic Acid Tield a s % Si02 s = 6.5 X = 8.0
= 3 5
37 40
400
00 73
73 80
In general, a somen-hat higher recover). of silicic acid of IoLvor molecular vieight is obtained x h e n the tert-butyl alcohol is added to the silicic acid sol immediately after its preparation and thc mixture then permitted to stand in order to alloiv the silicic acid to polyinerize in the presence of the alcohol. However, in this case it is not possible to folloiv the polymerization by the titration procedure dcscribcd because of the prcsence of the alcohol. ESTERIFICATION O F SILICIC ACID
The freshly prepared alcoholic solution is a clear liquid having osity little higher than tei-t-butyl alcohol. I t contains a small amount of mineral acid, corresponding to a p H of 2, whivh appears to have a stabilizing effect, since attempts to lowcr the acidity by adding small amounts of neutralizing agent caused more rapid gelling. If held a t ordinary temperatures (20-30 ' C . ) the extract sets to a hard, clear gel in 2 to 4 da be improved by diluting the solution with ad alcohol or cooling it to -2O'C. Hoxever, a much greater degree of stability is obtained by removing residual m t c r , either with desiccants or preferably 6y vacuum distillat ture, separating thc. \r-atcr as the alcohol-\vat
To 1540 cc. of a solution of silicic acid in tert-but~-lalcohol, prepared as described, an equal volume of n-butyl alcohol is added together with 6 gram5 of barium chloride. The purpose of the latter is to rc,place the small amount of sulfuric acid present in the extract with an equivalent amount of hydrochloric acid by the precipitation of barium sulfate. The mixture is placed in a 3-liter flask fitted with a 1- or 2-inchtiianieter fractionating column about 2 feet long having sufficient rondenscr capacity topermit as much as a 4 : l k f l u x ratio. Distillation is carried out at 30 mm. mercury pressure with a reflux ratio of less than 1: 1 and a t a rate sufficient to remove 1131 cc. over a 2-hour period. During this time the original volume is maintained in the distilling flask by the further addition of 71butyl alcohol. T h e temperature in the distilling flask during this step rises from 28" to 30" C. Distillation is then continued a t 60 nini. mercury over a period of 5 hours, during which time 1606
S = 6 + C
where C = intercept on diethyl Carbitol axis This X value then ranges from 0 for gelatin solutions alone (or for silicic acid sols of estremely low molecular weight) t o about 11 or 12 for silicic acid solutions rr-hich have polymerized to the gel stage and 11-hich, therefore, contain material of very high molecular vieight.
The vslueu in Table I show the increa$e in the X value ir-hich occurs as an aqucous silicic acid solution, prepared as TABLE I. S VALUEOF Y r r x I c AcrD SoLmros (65,SiOii AGED described, is permitted to age a t pH 1.7 and a t 12", 25", and A T pH 1.70 -1-8" C., respectively. Hours A t I?' c. . i t 25. c. . i t 48O C If the aqueous silicic acid solution is permitted to age 0 3 0 3 IO 3.0 for different periods of time before the tert-butyl alcoliol 4 :3 7.8 .i li is added and thc alcohol solution is then salted out within 7 1) "0 6 3 !I I about 15 minutes, the yield X3 8 n of silicic acid increases with the . I' value as follows: OF TYPICAL BATCHES OF Poi2i-siLrcrC A m ) I'LTER TABLE 11. PROPERTIES S Value oi Sol Yield as Sios, % Extraction Gel ,
~
3.0
5 0
8.5
37
53 82
Code
~~
Age of Yield aqueous of soln , h r . SiOv, CC
738-401
The ratio of the volumes of tert-butyl alcohol to silicic acid solution used has only a minor effect on the overall recovery of polysilicic acid. For example, in the case of the silicic acid solutions aged a t p H 1.7 a t 12" C. to various X values prior to
-41:
-42' -43 J 420-63'; -64, 420-701 414-611
420-66 420-68 698-119'$ -122,
"
"
Distn Time. H r . 55-65 mni. 760 m m .
30 m n i .
'
0.0
30
2 0
2 5 16 5 37.5
58
1.8
83
1.2 1.5
0.5
87
45
2.5
[ 4.8 113.3 120 8 I37 3
2.3 1.8 2.2
12 'I 2
... ,
.inalysis, CC H20 Si02 0 14 0 12
:,E:;
,.
... ... 16.
:;;
;i,
::;;
, . ,
..,
..
" '
015 0.15
19 5 19 8
2':
19 0 6 20 23 36 20 0 1 9 i 23 5
BuOi'Si 0 31 0 38 0 43
o.0 0 26
20 2
Time ;it
903 C , Hr.
330
0 48
68 100 >200 >ZOO
0 33 0.59
30 >IO0
1.52 >1500
0 34 0 48 0 35 0.29
31 >lo0 20 6 17 3 32 80
63 12.50
... ,
Renoerie Cornpatibility
0 $4
380 450 ii0
53
16 72 168
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1382
:c. of )i-butyl alcohol are added and 3260 cc. of distillate rei l l tlie flask rises :ram 39 t o 60' C. At this point the solution in the distilling flask has a ~ o l u m e Jf about 1510 cc. and contains approximately 1 5 r i Si& About 12 g r a m of Dicalite filter aid are added, and the liquid is filtered to remove precipitated salts and traces of silica gel. Distillation is then continued at atniosphcric presaure with a reflux ratio of a t least 4:l after addition of 485 cc. o t ri-butyl rleohol, tlie distilling rate having been adjusted so as to remove 7 5 2 cc. of distillate over a period of 5 hours. The tcnipcrnturc in the distilling flask finally reaches about 121' C. The 1210 cc. of product a t this point are then niiscd with 10 grams of Dicalite and 20 grams of Darco decolorizing carbon rtncl sgain filtered. A typical product has the following analysis, aftcr having bee11 adjusted t o about 2 0 7 SiOs by the additioii of anliytir~iustihutyl alcohoi: moved. During this operation the tempcraturcn
ii&
C;.
Water. c. IUO c r suln.
ciliolllle,
9 €1
L
20.11
0 11 0.03 3.8
Total julids, ( ; .-\nalysis of solids, r; Si02
C
Degree of esterific,arion, BUO/Sl
38,YS SY.0
28 98 0.613
Vol. 39, No. 11
tatively from the container in \rliich the evaporation out.
i3
rarried
E F F E C T S OF IYCREASING DEGREE O F ESTERIFICATIO?.
.I? indicated i n Table 11, continucd distillation at 6 3 mni. pres,sure over a pc'riod of 37 hours reducc.~6he water contc.iit of tlie wlution of polydicic acid ester in n-butyl alcohol from 0.14 to 0.023'7r. -4t the same tirile the degree of esterification inrrcasea fi,oni 0.31 to 0.48 butoxy groups per silicon atom. This increase, in the degree of esterification results in improved conipotibility with a Iiydrocnrbon solvent such as benzene and also incrc-aws thc -tal)ility of the saniplc toivard gclling in the acceleratt,cl aging tc3t a t < N oc. The stability u i the resiiltirlg product toivard gylling ~vIii1n iiratcd a t 9'3' C. is reduced, for a definite degree of estc,rifieation, :IS thc aqueous silicic acid solution R i aged prior to thc cstraction Step. Tlii-: ran he noted by comparing the sampl(.- 11:iving a degrw of oslc:rification between 0.30 arid 0.35. \-ISCOSITY OF B U T Y L POLYSILICATE SOLUTIOS
hIETIIODS OF ANALYSIS
SILICA. A 2- to d-graril sample of tlie solutioii of cstei.ified polysilicic acid is neighed into a tared, covered platinum crucible, ::vaporatt.d slo\viy to dryness on a steam plate, iiiuistened with :t ie\v drops oi 1: 1 sulfuric acid, and ignited sloivly i n t i illriffle ior 1 hour. The crucible is weighed, 4 to 5 drops of 1 : l sulfuric acid and 10 cc. of hydrofluoric acid are added, and the iiiixrure is heated gently on the stcam plate, evapbrated to d r y -*oolcd in a desiccator, aiid weighed. The loss iii 'reatnient n-ith liydrofiuoric acid is equivalent to tho ivvight of d i c a iii tile sample. CHLORISE. This is diitermined b v evaporating a weighed saniplc to drynebs, decoiiiposing the re&uc Tl'ith so&uni percyxide !ii a Parr bomb, and determining chloride in the residue gravimetrically as silver chloride. ~ V A T E R . Five cc. of the polysilicic acid ester solutioii is iiieas:ired into a 125-cc. Erlenmeyer flask arid diluted n-ith 10 cc. of anhydrous methanol. \\'ater is titrated by the Fischcr nietliod J!;
1w.1~ SOLIDS. Because of the resinous nature oi the dried Droduct, tlie solvent caii best be removed by evapuration from a thin filni under reduced pressure. A measured volume (3-5 cc.) of solution of butyl polysilicate is placed on a tared 3 X 5 inch glass plate in a desiccator through which perfectly dry air is drawn for 24 hours a t 0.5 mm. pressure, and the weight of the residue is de tcrniined. pH, The pH of the mixture is determined by mixing a sample with an equal volume of distilled water, shaking until a thorough dispereion is obtained, and measuring the pII with a glass electrode. STABILITY. T h e stability of the solution of polysilicic acid ester is determined by sealing 5-grain samples in thoroughly cleaned 6 X 6,'s inch Pyres test tubes and immersing the samples in a steam bath a t 98-99" C. until the gel point is reached, at which the liquid will no longer flow. This is taken as the gel time. I t is important t h a t the test tubes be cleaned in a chromium oxide-sulfuric acid cleaning solution, rinsed at least a i s tirnes with distilled water and three times with acetoile, and tlieri dried a t 110" c. COXPATIBILITY WITH BESZEXE. One cc. of a polysilicic acid ester solution containing 20y0 SiOpis measured into a 10 X 1 inch test tube. Benzene is added from a buret until a slight turbidity appears, and the result is espressed as the volume of benzene in ec. required to reach this end point. However, this test is applicable only to samples which have been esterified t o a low degree. Highly esterified products are completely miscible with benzene. DEGREE OF ESTERIFICATIOS.h weighed sample of the solution is evaporated in a thin film on the inner walls of a 1-liter flask subjected to vacuum (2 mm. pressure) and heated externally by live steam. The dried residue usually tends to flake from the sides of the flask and is easily removed. Heating a t 2 mm. pressure is continued for 2I/y hours in order to ensure complete removal of butanol. T h e residue obtained in this manner, or t h a t obtained in the determination of total solids, is analyzed for carbon by combustion and for silica as described. The degree of esterification, calculated from these data, is expressed as the ratio of ester groups t o silicon atonis. If silica and per cent carbon are determined o n the same residual solids, it is not necessary to use a weighed sample of the solution or to transfer the residue quanti-
'Thc approsimatc viscosity of a typical n-butyl alcohol s o h l i o n r i f polysilicic acid cster n a s dctcBriiiined over a range of conci~ntrutiiinsto learn \\-hcthcr the molecular w i g h t coultl tw dctcr~ mrthod. I I o w v e r , as s h o w i below, :hc, rcwlts 1iiiritt.i I Jthis iiidirati. that the niolecules may be roughly spherical 1:itticr than linear, arid therefore molecular weight cannot be detc~ririinr:tl by thc Stautlinger equation. .-I~ m i p l eof butyl polysilicate ( 2 O . z f ; SiOz) in n - h i y l alrohol (So. 733-13, Table 11) was diluted t o several lower et,ncoritraticins with the same solvent, the visrosity a t 25" C. determined liy iiiiwis of Ostivald pipets, and the specific viscositv ralculated from the equation Vs VBD = -1
VQ
where vas = spccific viscosity q a = viscosity of solution as measured by O s t d t l pipet = viscosity of n-butyl alcohol ,
The rcsults an' s h o ~ i in i Talilr: III.
TABLE 111. VISCOSITY 9x1 D E M I T YD.~T.< &Oa, % by l \ - t .
Ostmald Pipet
drc. d t 29.5" C.
df
Density 2Y.jo C.
As Till lw slio\vn, thesis d:ita a p p w r to coniurm with the. equation suggested hy G u t h , ( h i d , :rnd diniha ( 7 ) for a diipersion of spheres: VSP = 2.5, t l-I.lc* where c = volume fraction of suspendod spheres or dispersed phase, cc./100 cc. I t is therefore teritatively concluded that the polysilicic acid units niay be roughly spherical in shape, since linear or elongatctl moloculw would not show this viscosity behavior. Although the value of c cannot be measured directly, it is possible to estimate its value indirectly if one assumes t h a t the total volume occupied by the solute is directly proportional t o the silica content of the solution and t h a t the volumes occupied by the solvent and solute are additive. Thus from the assumption that (volume of solute) +'(volume of solvent) = (total volume), for 100 grams of solution one can write:
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1947
-a + s - =100 - -(1
A
hcrc
,q = =
(1
=
0.802 I)
= =
100
as
D
0.802
r y by weight of SiO, in solution by weight of solute, a being the conversion factor
P
ririknonn density of pure solute iitlnsity of n-butyl alcohol solvrnt iitLnsity of solution
f2) t
I1 =
1
-
0.802 0.006S:-*C
1383
solute m:ty he :tliout 2.2 x 20.2 = ,44(.5j% by ni~iglit,although t h e t l J t : i l >olidi, :iftc.r bcling dried under vacuum, amounted t o only 32 10 3jcC. It is therefore probable that in thc solution the polysilic*ic ester is associated with a certain fraction of the T I butyl alcohol solvent. One possible oxplanation is that the roughly *phericnl miilecul(~sof I~olysiliticester l i a i ~a highl,v 1ir:inchid atid open structurc within which ,wlvc.nt is hc~ltl uic~clianic:illy. The> hut yl po1ysilic:itc wlution also contains clii~riiicallyhound n-atixr (prolrably in the ioi,ni of une?t trifid ,sil:inol, ---8iOII? group.) \\-liic.h is pxi,tially 1ihr:ratcd upon cvaporation of'rhe solvent. I'lJt' i ~ s n i n p l ~i ~ ,it Iii. simple 733-43 uird for i i i i ~ ~ ~ . ~ i i ~ ~tlic. ~ ~ ~free - ~ iw ~ ~ t tt, ir t, . by ~ , titration \vith the rc.:ig!'cziit.aniouiited to ohly 0 . 0 2 3 5 : yet in tlie butanol rcw)vcred from :i driod saiiiple 2.30': \v \ r f t h e o1,iginal solution. C u b Ic-itlut, show tli:it iinotlic,r 1.33 r l i c ~n.c,iglit ( i f o r i g h t l wlutioii I 1v1iic.h i. probsbly still p r w ' n t :is i i r i ~ o n d ~ ~ ! i .>il:itiol id group-. Tliu. t lic empirira! r,oinpoqit ion of ' h C .IJlUtt' 111;1y IJ?:
13; ~,~,.uniitig V U l U i i l l ' i :ii'e addiTiw, tlic volrirnc of 44.5 gr:trns of tliis solri1r \voulJ l)c :itmiit (20.2 2 . 2 ) 1 I. I ,'0.7fj7 3.7 (!I,:, 0.902) = 30.2 cc. : t1ii.s coi pond. t 0 ii tli.nsity of 1.13 gr:tniy pc'r re., which is within tlic range of th(b value 1.17 * 0,08 c7dcu1atc:d
+
+
+
from il(,naity and viscosity d a ~ a .
I'ROCI-JI?I-RE. .I (1.112-gi~iiii sample of solution 733-43 was 1, \re11 dried, round 250-cc. Pyres flask connected ce trap to a vacuuni p u m p . The flask \vas parin water at 33-40' C. arid rotated 21.5 the solvent ill order to deposit t h e re&lue as a thin film on the nalls. After 31 ' 2 arid 4 hours, rwji vely, dry air was led into ighrd. Only 22 mg. were the flask. which w i s stoppered and lost in the last half hour. I!ii?idue wiglit = 3.130 grams, or 34.6:~; of the oi,iyinal \v-eiglit. The rondensate i n tlic t r a p contained 0.218 gram water by titration. o r 2.30C; 011 the original sample \wight. .\nalysis of tlie residue gave silica, 63.30; carbon, 25.01 ; and hydrogen, 5 . 2 8 5 , From the carbon and silica analysis the ratio of (C4fI90),'Si= 0.505, which is i l l good agreement with 0.48 found by previous analysis. Hon ever, the hydrogen corresponding trJ C I & would amouiit t o orily
The differcnce of O . U $ is greatrr than t:\perimental error and is believed due to -SOH g r o u p in the dried samplr, On the original sample m i g h t this irould amount to
SlO!.
T.IBLEIv. 5
1.13 2.62
CALCUL-kTED Y A L C E S O F C AYD 12 %P
C
0 067 0.129 0.270 0 813 2.540
5.18 9.68 20 20
From r t i v relation
=
0.685=
0,022 0.041(;1) O.Oi.5 0.138 0.335 Average a =
5
2.2(lj 2 3(0)
2.115)
2.0(7) 2.1(2) 2.2
ad ((1 - 0.802), t a k i n g a = 2 . 2 ,
d = 1.17
*
0.08
The "onstancy Of t h a t the csperimcntally determined viscosity-concentration relation approximately follows the rhtwrtical equation for spherical particles in suspension. B U T Y L POLYSILICATE I N SOLUTION
The value a = 2.2 suggests that, in the original concentrated jolution containing 20.2% SiO,, the concentration of actual
- 1.35 = 3.74% water; this corresponds to (3.74,'91 X (60, 20.2) = 1.23 unesterified hydroxyl groups, together ivith about 0.5 butoxy aroup per silicon atom. Itis-recognized that the analysia for hydrogen is not sufficiently accurate to justify such detailed calculation, but this and similar analyses lead to the general conclusion t h a t i n polysilicic acid there is about one hydroxyl group per silicon atom n-hich is less readily estcrifiable than the remainder. Thus the original solute must have contained 2.30
CIIANGES IN W.ATER C O N T E S T DURING STABILITY T E S T S
The presence of unesterified hydroxyl groups is also indicated tis' the observatioIl that, solutions of butyl polysilicate are heated in sealed tubes a t 100' C., the water contrnt at first rises, inthen diminishes t o a new minimum, and thcreafter again creases. During the second rise in the water content the viscosity of the solution also begins to increase and thereafter continues t o rise until the gel point is reached. This behavior is indicated in Table V.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1384
Vol. 39, No. 11
may lie due to thtx further rnt~c~lianical entnnyl~~mcnt of u - ~ i u t y l Table I-, CHAXGEIS CONCESTRATIOS OF WATERIS TYPICAL alcohol s:olveiit within the, growing polysilicic w t c r molcc*ulnr SAMPLES units. IIr. Heated 0 10 20 30 40 50 65 iG
Degree of Esterification, BuO/Si = 0.48 Viscosity. CP. HzO, % 1 21 0.07 ... 0.16 ... 0.25 1.21 0.24
...
n . 18
1.21
O,l4 0.22 0.26
, . I
1.23
Degree of Esterifiration, RuO/Bi = 0.31 Viscosity, CP. H20, % 1.23 0.14 ... 0.25 1.23 0.33 1,233 0.20 . , , 0.23 1.25 0.30 ... 0.31 1.28 0.34
I t i, postulated that, the unestei,ified hydroxyl groups ivitliin the polysilicic acid molecule slowl>-undergo intramolecular condensation; thus they liberated water and account for the initial rise in the n-ater content of the solution. This water may then be consumed in the hydrol! of butoxy groups, liberat iiig ributyl alcohol and forniing nelv hydrosj-1 groups, al~iioughthis second increase in hydroxyl groups has not bccn checked by actual analysis. This process r o u l d lead t o a decrease in the water content of the system. Finally, the nen-ly formcd hyclrosyl groups may then permit intermolecular condensation anti the growth of larger molecular units, water again being liberatetl in the process, and the viscosity increasing. This increase in viscosity
ACKNOWLEDG3IENT
Thc authors tliank those n h o have cmtrikiutcd t o : h i 3 I W ~ ~ ~ L ~ with sptLci:tl acknowledgmt~nt t o J. S. Iiii,k, II. 13. I~t~rriaId, I
